Field of the Invention
This invention relates to a color-forming composition. More specifically, it relates to a color-forming composition comprising a leuco dye, an aromatic amine or its derivative and an organic solvent.
The primary object of this invention is to provide a colorforming composition which gives a color of improved fastness and excellent discoloration resistance, upon contact of a leuco dye contained therein (which will be referred to as a "color former") with a color developer. The above object is attained by using an aromatic amine or its derivative as one component of the color-forming composition.
Description of the Prior Art
Many instances of application of color-forming reactions between color formers and color developers to recording methods have been known.
For instance, there are adopted methods comprising transferring a solution of a color former to a color developer sheet, and a method comprising coating a color former solution with a wall of a high molecular weight substance, rupturing the resulting microcapsules on a color developer sheet and thus transferring the solution to the color developer sheet. Many forms of both reactants have been proposed and are known in the art. For instance, there may be mentioned a recording sheet prepared by dispersing a solution of a color former in a binder and coating the dispersion on ordinary base paper or a color developer sheet, a recording sheet prepared by coating microcapsules such as described above onto base paper or a color developer sheet (the so-called pressure-sensitive recording or copying paper), or a printing method comprising feeding a color former solution in the form of an ink composition to a color developer sheet through a stencil. These proposals are in common with one another in the point that both the color former and color developer are used, though they differ in the manner of using these reactants.
Examples of the color former include triarylmethane compounds, diphenylmethane compounds, xanthene compounds, thiazine compounds and spiropyrane compounds.
The color developers include, for example, clay minerals such as acid clay, activated clay, attapulgite, zeolite or bentonite; organic acids such as tannic acid or gallic acid; and phenol resins.
These agents are specifically disclosed, for instance, in U.S. Pat. No. 2,505,489, U.S. Pat. No. 2,548,366, U.S. Pat. No. 2,730,456, U.S. Pat. No. 2,730,457, and laid-open Specification of German Patent Application No. 1,919,397.
In general, colors formed by the reaction between the above two reactants (color former and color developer) are very inferior in fastness to those produced by ordinary pigments. This tendency is especially outstanding when the resulting colors are exposed to sunlight or fluorescent light. The fastness of colors depends on the structure of the color former, and varies greatly according to it. Colors formed by certain color formers degrade under the influence of light and color hues are changed. Many color formers useful for the above-mentioned recording methods are included within this type.
The difference among individual color formers with respect to the color fastness, or the absence or presence of degradation in formed colors brings about a great disadvantage in colored images of blue, blue black, green and black colors, especially colored images of black, because colors of images are easily degraded with the lapse of time or under irradiation of light, exhibiting a very low color fastness.
Among color formers used for forming a color hue comprising a blue color component, such as blue, blue black and green, most of the blue-color-forming compounds other than the thiazine compounds do not give a color fastness sufficient for practical use. Accordingly, when colors formed by employing such color formers are allowed to stand indoors or exposed to sunlight, the color density is readily lowered or the initial color hue changes readily, with the result that the color intensity is reduced.
For instance, a combination of Crystal Violet Lactone and benzoyl leucomethylene blue is mainly used for forming a blue color. However, since Crystal Violet Lactone is much inferior in resistance to light, the color formed easily disappears when allowed to stand indoors or exposed to light, with the result that the once formed color is changed into a color derived only from benzoyl leucomethylene blue (light blue) and the commercial value of the colored image is therefore extremely lowered.
The fluoran compounds proposed in the specification of U.S. Pat. No. 3,501,331 are used as color formers for black, green and blue black colors. Although these compounds form a bluish green color upon contact with color developers, the formed color changes to red when the colored image is allowed to stand indoors or exposed to sunlight. Accordingly, black, green and blue black colors formed by employing such fluoran compounds become reddish when allowed to stand indoors or exposed to sunlight, and therefore, the use of such compounds can not be free from disadvantages such as the change in color hue and the reduction of color fastness.
The fluoran compounds disclosed in the specification of British Pat. No. 1,168,455 are used as color formers for a red color. However, the fastness of colors formed by employing such compounds is inferior to the color fastness obtained by using thiazine compounds, and the light resistance of these fluoran compounds has been required to be improved.
Various attempts have been made with a view to producing color formers free of the above-mentioned defect. The present inventors have made investigations to overcome these defects from a view-point quite different from that of the conventional art.